electrocardiographic rate patterns · 2007-08-24 · historical information of previous fits,...

Br HeartJt 1981; 45: 281-91

24-hour electrocardiographic study of heart rateand rhythm patterns in population of healthy childrenD P SOUTHALL, F JOHNSTON, E A SHINEBOURNE, P G B JOHNSTON

From the Department of Paediatrics, Cardiothoracic Institute, London

SUMMARY Twenty-four hour electrocardiographic recordings were made on 104 randomly selected,healthy 7 to 1 1-year-old children. Ninety-two were technically adequate and suitable for analysis. Themean highest heart rate measured by direct electrocardiographic analysis over nine beats was 164 ± 17.The mean lowest heart rates were 49±6 over three beats', and 56±6 over nine beats' duration. Themaximum duration of heart rates <55/minute was 40 minutes. At their lowest heart rates 41 children(45 per cent) had junctional escape rhythms, the maximum duration of which was 25 minutes. Ninechildren showed PR intervals >0-20 s and included three with Mobitz type I second degree atrioven-tricular block. Nineteen (21%) had isolated supraventricular or ventricular premature beats (<1/hour).Sixty subjects (65%) had sinus pauses that could not be distinguished on the surface electrocardiogramfrom those previously described as sinuatrial exit block or sinus arrest. The maximum duration of sinuspause measured over 24 hours on each child was 1-36±+0'23 seconds.Thus apparently healthy children show variations in heart rate and rhythm over 24 hours hitherto

considered to be abnormal.

Several recent reports have ascribed a number offits, faints, and attacks of dizziness in childhood toabnormalities of sinus node function,1-4 a diagnosisoften made from the interpretation of patterns onthe surface electrocardiogram alone.5 The supposi-tion, however, that these patterns were abnormalhas been made in the absence of adequate long-termelectrocardiographic data on the incidence of suchpatterns in healthy symptom-free children withoutheart disease. This study of 24-hour recordings ofthe electrocardiogram in a randomly selectedpopulation of apparently healthy school childrenprovides normal data with which suspected ab-normal electrocardiographic patterns can be com-pared.

Patients and methods

One hundred and four apparently healthy schoolchildren between the ages of 7 and 11 years (mean9-4 years) were randomly selected for 24-hourelectrocardiographic recordings from the register ofa primary school in Dorchester. The school servedboth an urban and a rural community and containedchildren of all social classes.Twelve recordings were rejected because of poor

Received for publication 23 June 1980

quality. Five had undetected intermittent leadfractures, two had electrode displacement, in four afault in the cassette resulted in jamming of the tape,and in one oxide coating of the recording head pre-vented signal reception. Forty-nine recordings ofgood quality came from boys and 43 from girls.Eleven of the girls, but none of the boys, werepubertal.A questionnaire was sent to the parents requesting

historical information of previous fits, faints, or"funny turns" or of other serious illness, and wascompleted in all cases. A full clinical examinationwas also performed. Standard electrocardiographicrecordings were not made.Each child was fitted at school with a 24-hour

electrocardiographic* recorder and this remained inplace until the following day. Normal activities wereencouraged throughout the recording. An attemptwas made to document diaries of activity during therecordings but notes kept by the children were ofteninaccurate and these were therefore abandoned.

Recordings were analysed directly using a systemthat automatically detected heart rate or rhythmpattems outside preset standards. On detection ofsuch an abnormality the preceding 30 seconds andsubsequent 10 seconds of the electrocardiogram*Oxford Medical Systems.

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I

Fig. 1 Twenty-four-hour

electrocardiographic recording

(including a timed reference

signal for accurate analysis o.fheart rates) showing a sinus

tachycardia of 180/minute in a

healthy 9-year-old boy on

exercise.

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were displayed. The recording and analysis system

incorporated a synclock mechanism which, by

providing and processing a reference time signal,

ensured that possible variations in tape speed did

not produce artefacts of rhythm or rate.

The highest and lowest heart rates occurring in

each child over a 24-hour period were measured

from the electrocardiographic print-out after direct

analysis of the whole of the recording from 3, 5, and

9 beats or 2, 4, and 8 RR intervals, respectively

(Fig. 1).

A search over three randomly selected hours of

,daytime (10-00 to 18&00 hours) and night-time

(00-00 to 06&00 hours) recordings in 26 subjects

with bradycardia below 55/mmn was made for the

frequency of episodes of bradycardia at each of

three levels: <55/minute, <50/minute, and <45/

minute (as measured over nine or more beats). The

maximum duration of bradycardia <55/minute

was also measured in these 26 subjects. The presence

,of junctional escape rhythm, when present over

three or more beats, was also noted (Fig. 2), and the

-frequency and maximum duration of this rhythm

,over the 24 hours of recording were measured in

24 children with episodes of junctional rhythm.

In each child the 24-hour electrocardiogram was

-examined for the following PP interval patterns.

(1) A gradual increase and decrease in PP intervals,

(usually termed sinus arrhythmia).6

Fig. 2 Twenty-four-hour[

recording showing an episode ofjunctional rhythm at a rate of>f.37/minute (as measured over

-three beats' dutration).Ui

(2) A sudden increase in PP or PQ interval ex-

ceeding the previous PP interval by more than

110 per cent (a pattern usually described as

resulting from sinus arrest (Fig. 3)).

(3) A sudden increase in PP interval exceeding the

immediately preceding PP interval by between

90 and 110 per cent; an electrocardiographic

pattern that has previously been described as

probably resulting from sinus arrest. Classical

Mobitz type II second degree sinuatrial block7-9

(Fig. 4) would also result in such a pause but the

sinus interval subsequent to the pause should be

equal in length to the pre-pause interval. This

latter situation, however, may be modified by

autonomic tone and it is possible that this sudden

doubling of PP interval may be described as

second degree sinuatrial block rather than sinus

arrest.

(4) A progressive decrease in PP interval ending

with a sinus pause exceeding the previous PP

interval by between 50 and 90 per cent (the

pattern described as representing probableWenckebach second degree SA block).7-9 This

pattern was only counted when the calculations

of PP increments and the graphical differentia-

tion from sinus arrhythmia could be made'0

(Fig. 5). Fig. 6 on subject A shows a PP interval

pattern previously labelled by other authorS7-9

sinus arrhythmia and Fig. 7 on a different

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24-hour electrocardiographic study of heart rate and rhythm patterns in children

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Fig. 3 Twenty-four-hour electrocardiographic recording showing a sudden prolongation of PP interval (1120 ms)exceeding the previous PP interval (480 ms) by more than 110 per cent. This pattern cannot be distinguished fromthose previously described as probably representing sinus arrest. Temporary complete sinuatrial exit block with highatrial escape (P wave indistinguishable from sinus) or with a variation in timing of sinus impulse generation causedby autonomic tone could also be described as a possible cause of this sinus pause.

subject, "B", shows a PP interval patternpreviously called by other authors10 Wenckebachsecond degree sinuatrial block.The frequency of patterns (2), (3), and (4) in the

total population of children is shown in Table 4.The number of episodes of patterns (2), (3), and (4)in a subgroup of 30 children with these patternswas also measured (Table 2).The maximum sinus pause (the longest PP or PQ

interval) was measured from the 24-hour electro-cardiogram of each child.

Finally, all recordings were analysed for thepresence of premature beats (number per hour) by adirect count from six randomly selected hours ofrecording.

no difference in heart rate values or rhythm patternfindings between this group of children and thosewho had not fainted (Table 5).Mean values for the highest and lowest heart

rates over 24 hours in each subject are shown in~~~~~680 710 1480 l

Results

In all cases a full clinical examination failed todetect evidence of structural congenital heartdisease or of any other serious illness. No childstudied felt dizzy or fainted during the course of the24-hour recording.Ten of these randomly selected, healthy children

have a history of previous fainting episodes butnone of the children had regular faints. There was

Table 1 Heart rate from 24-hour electrocardiogramin 92 children

Highest rate Lowest rate

Measured Measured Measured Measuredover 9 beats over 3 beats over 5 beats over 9 beatsor8RR or2RR or4RR or8RRintervals intervals intervals intervals

Mean±SD 164±17 49±6 52±6 56±6Maximum 195 64 68 69Minimum 130 37 38 42

'720 720 720 1400

Fig. 4 Twenty-four-hour electrocardiographicrecordings from two subjects showing sinus pauses with asudden prolongation of PP interval exceeding theprevious PP interval by between 90 and 110 per cent.This sudden doubling of PP interval on the surfaceelectrocardiogram has been previously described asresulting from sinus arrest. Classical Mobitz type IIsecond degree sinuatrial exit block could also result insuch a pause but the sinus interval subsequent to the pauseshould be equal in length to the pre-pause interval. Thislatter situation., however, may be modified by autonomictone and it is possible that this sudden doubling of PPinterual may result from second degree sinuatrialblockcrather than sinus arrest.

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Southall, Johnston, Shinebourne, Johnston

Second interval ( ms)540 700 860 10204 620 i 780 t 940 ~ 110

620

700

780

860

940

1020'

1100'

1180-

620

700

780'

860'

940'

1020'

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Table 1. Lowest and highest heart rate histogramsdid not significantly differ from those of a normaldistribution (proven by the Kolomogorov-Smirnov

)01180 test"). All children showed at times a phasic varia-tion in rate, normally termed sinus arrhythmia.6Abrupt changes in heart rate were also a frequentfinding (Fig. 8).

Table 2 shows the following.(1) The number of episodes (over 24 hours) of

junctional rhythm, in a subgroup of 24 subjects.(2) The number of episodes (over 24 hours) of

the different PP interval patterns, in a subgroupof 30 subjects.

(3) The number of episodes (over 3 hours of day-* time and night-time recordings) of heart rates

below 55/min, below 50/min, and below 45/min,in a subgroup of 26 subjects.The mean longest sinus pause occurring over 24

hours for each child was 1-36 ±0-23 s. The overalllongest pause seen in this population was 1 88 s.The maximum duration of junctional rhythm

and episodes of bradycardia below 55/min over 24hours in the subgroups of 24 and 26 subjects re-spectively are shown in Table 3.The frequencies of junctional escape rhythm, PP

interval pattern variations, premature beats, andatrioventricular block in the total population ofchildren are shown in Table 4.Nine children showed intermittent PR prolonga-

tion >0-20 s including three with Mobitz type 1second degree atrioventricular block (Table 6)(Fig. 9 and 10).

Isolated superaventricular premature beats wereseen in 19 children (21 %), but none had more thanone an hour. One additional child had isolatedventricular premature beats (again less than onean hour).

" Discussion

Fig. 5 Two graphs showing a plot of PP intervalson the Y axis against succeeding PP intervals on theX axis. The line at 450 shows the position of PPintervals of equal duration. Graph (a) from subject Ashows the pattern previously called sinus arrhythmia.Graph (b) from subject B shows the pattern previouslylabelled sinuatrial Wenckebach block.

Analysis of tape recordings of the electrocardiogramhas proved a valuable technique for the investiga-tion of heart rate and rhythm over long periods oftime. Care must be taken, however, in comparingdata over 24 hours with data from relatively briefstandard electrocardiograms. Thus a range of heart

800 820 940 1180 820 760 720 700 680A

Fig. 6 Twenty-four-hour electrocardiographic recording on subject A showing a phasic variation in heart ratepreviously termed sinus arrhythmia.

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Table 2 Number of episodes ofjunctional rhythm, PP interval patterns, and episodes of bradycardia in subgroupswith these findings on 24-hour electrocardiogram

No of episodes Junctional rhythm Sinus pauses Rates below 55/min Rates below 50/min Rates below 45/min

A* B* C* Day Night Day Night Day Night

1- 5 4 1 3 7 2 8 1 11 36-10 5 4 6 1 5 2 111-15 1 3 7 2 216-20 1 1 3 1 121-25 3 1 326-30 2 1 231-35 4 1 136-40 1 2 141-4546-50 1 1>50 13* 3 1 1 1 1

Total no. subjects studied 24 30 26

*Present at all rates below 60/minute.A*, PP or PQ exceeding previous PP by 50 to 90 per cent with incrementally decreasing PP before pause.B*, PP or PQ exceeding previous PP by 90 to 110 per cent.C*, PP or PQ exceeding previous PP by > 110 per cent.

rate from 37 a minute to 195 a minute was foundover 24 hours in this population of healthy children.Previous studies of heart rate variation in childrenat this age using standard electrocardiograms haveshown, not surprisingly, a much narrower range(55 to 115 for 8 to 12-year-old children).'2 Abruptchanges in heart rate were also a frequent findingover 24 hours and may represent the effects ofvariations in autonomic tone. Alternatively, sinustachycardia of sudden onset may result from so-called sinus node re-en:ry.13 14 Long-term electro-cardiographic recordings, though recommended,4have not previously been performed on healthychildren. It is interesting to recall, however, thatHolter monitoring of 50 healthy medical studentsaged 23 to 27 years showed a similar, and un-

expected, variation in heart rate and rhythm.'5Irregularities of sinus rhythm have been de-

scribed either as sinus arrhythmia6 16 (a "normal"finding said to be sometimes related to respiration)or as a manifestation of abnormal sinus nodefunction. An accurate diagnosis of sinuatrial exitblock or sinus arrest can, however, only be made byrecording potentials directly from the sinus node.Though it has been stated7-9 that measurement ofintervals between atrial potentials, on the surfaceelectrocardiogram, may be used to diagnose exitblock such measurements may be complicated byother factors. Thus, in electrocardiographic patternsdescribed as sinuatrial block exact whole multiplesof PP intervals may be disturbed by autonomiceffects or by escape pacemakers whose inherentrates are only slightly slower than that of the sinuspacemaker. Thus, we have not attempted to define,electrocardiographic patterns of exit block or sinus

arrest but instead have measured PP or PQ intervalsand categorised pauses arithmetically into groups.

On the basis of this categorisation, and consideringthe effects of autonomic tone and escape pacemakeractivity, we contended that it is impossible to dif-ferentiate on the surface electrocardiogram thepauses we have described from those previously de-scribed as sinuatrial exit block or sinus arrest.Whether the primary mechanism of the pauses issinus arrest, sinuatrial exit block, or the effects ofautonomic tone remains uncertain but the highfrequency of these pauses in healthy childrensuggests that the underlying mechanism is not ofclinical importance.

Prolonged sinus pauses said to result from sinus

Table 3 Maximum duration ofjunctional escaperhythm and episodes of bradycardia < 55/minute insubgroups with these findings on 24-hour electrocardiogram

Maximum duration J7unctional rhythm Heart rates lessof episode than 55/minute

1 to 10 seconds 1211 to 20 seconds 1321 to 30 seconds 4 431 to 40 seconds 141 to 50 seconds 151 to 60 seconds 21 to 2 minutes 63 to 5 minutes 1 16 to 10 minutes 110 to 20 mninutes 220 to 30 minutes 1* 1**

Total no. subjectsstudied 24 26

*25 minutes.**40 minutes.

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960 760 720 1000 960 800 680

680 1040 880 720 720 1040 960 760 640 1080

920B

Fig. 7 Twenty-four-hour electrocardiographic recording on subject B showing episodes of progressive reduction inPP interval followed by pauses exceeding the immediately preceding PP interval by between 50 and 90 per cent.The calculations of PP interval increment, according to Schamroth and Dove,'0 describe a pattern that has beencalled Wenckebach sinuatrial exit block rather than sinus arrhythmia.

arrest or complete sinuatrial exit block have beenreported.'7 In this study of healthy children themean longest pauses seen were 1-36 ±0-23. Perhapspauses in excess of 1 82 s (two standard deviationsabove the mean longest pauses) should be regardedas evidence for abnormal sinus node and escapepacemaker function at this age.

Sinus arrhythmia has been renamed irregularsinus rhythm by the World Health AuthorityWorking Party on terms relating to cardiacrhythm.18 Perhaps short sinus pauses, previouslylabelled sinus arrest or Mobitz type I and IIsinuatrial block, may also be more appropriatelyincluded in this latter definition.

Junctional rhythm may occur for two reasons.Firstly, the inherent pacemaker rate of the sinus

node may fall below that of the atrioventricularnode, the latter then initiating further impulseformation. This junctional escape rhythm may resultfrom normal sinus bradycardia or because of ab-normal sinus node function.19 Secondly, the in-herent pacemaker rate of the atrioventricular nodemay be accelerated.20 Junctional rhythm was seenfrequently in this population of normal childrenand we suspect that these episodes were related to asmall difference in intrinsic pacemaker rate betweenthe sinuatrial and atrioventricular node. The 95thcentile for maximum duration of junctional rhythmseen in this population was 20 minutes. More pro-longed episodes (of junctional rhythm) may indicateeither abnormal sinus node function or the presenceof an accelerated junctional focus.

Table 4 Rhythm patterns on 24-hour electrocardiogram recordings

Rhythm at lowest rate Sinus pausest Premature 1st degree A V block 2nd degree A V blockTotal no. subjects= 60 beats (PR 202Os)

Junctional Sinus A* B* C* 0 <1/h

No. of subjects 41 51 43 34 8 73 19 9 3(total n=92)

A*, PP or PQ exceeding previous PP by 50 to 90 per cent with incrementally decreasing PP before pause.B*, PP or PQ exceeding previous PP by 90 to 110 per cent.C*, PP or PQ exceeding previous PP by > 110 per cent.tSome subjects had more than one type of sinus pause.

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94/min 62/min

62/min 1601min

Fig. 8 Twenty-four-hour electrocardiographic recording showing abrupt changes in heart rate. After sinus rhythm

of 94 a minute there is an irregular slowing of heart rate to 62 a minute. The first two beats in the second panelhave an abnormal P wave and shortened PR interval preceding the QRS complex; they may represent junctionalescape beats. The subsequent tachycardia of 160 a minute is presumably sinus in origin. It is possible that a re-entry

mechanism may be responsible for this sinus tachycardia but only electrophysiological studies would be able to show

this mechanism.

Junctional escape rhythms and sinus pauses,

previously described as probable sinuatrial exit

block or sinus arrest, were therefore found in a high

proportion of this large, randomly selected, ap-

parently healthy, population of children. A know-

ledge of these normal findings must be taken into

consideration when surface electrocardiographic

recordings are used to diagnose sinus node dys-function at this age. Sinus pauses said to be ab-

normnal and to represent sinus arrest or sinuatrial

exit block, episodes of bradycardia below 50 a

minute, and junctional escape rhythms in children

with syncope or episodes of dizziness have led

others to a diagnosis of causative abnormal sinus

node function.'1 2 4 Many patients with sinus node

dysfunction have tachyarrhythmias in addition

to abnormal episodes of bradycardia ("the sick

sinus or tachycardia-bradycardia syndrome").2' 22

The degree and nature of both tachycardia (where P

wave discrimination is often difficult) and brady-

Table 5 Heart rhythm and rate variations in children with history offainting

Case Age Sex Lowest rate Premature Sinus pauses J7unctional Longest A V block No. of faintsno. over 24 hours beats A* B* C* rhythm sinus pause

(over 9 beats) (s)

1 9 M 51 0 + ± + + 1-40 - 12 9 F 50 0 + + - ± 1-44 - 13 9 M 62 0 - 1-20 - 24 8 M 50 0 + ±- ± 1-70 - 45 11 M 52 0 + -- + 1-52 - 16 11 F 63 0 - 1-28 - 17 11 M 56 + - 1-72 - 18 11 M 56 ± - 1-50 - 19 11 M 47 0 + 1-64 PRO028 110 9 F 55 ± + ±- + 1-32 PRO024 3

Mobitz I

Significance tests Mann Whitney x2 5-36 x2 0-014 x20.018 x20.193 x2 0-494 Mann Whitney XI' 0-346against 82 children who U test=343 NS NS NS NS NS U test NShave not fainted NS 275 NS

A*, PP or PQ exceeding previous PP by 50 to 90 per cent with incrementally decreasing PP before pause.B*, PP or PQ exceeding previous PP by 90 to 110 per cent.C*, PP or PQ exceeding previous PP by > I110 per cent.

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Southall, J7ohnston, Shinebourne, J7ohnston

P-R =024

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Fig. 9 Twenty-four-hour electrocardiographic recording from a healthy 9-year-old girl. The top panel shows an

episode of sinus rhythm with a PR interval of 0-24 s. The lower two panels show Mobitz type I (Wenckebach)atrioventricular block.

cardia regarded as abnormal must also take a

knowledge of normal heart rate values into con-

sideration. ReportS"3-27 on the results of surgery

for congenital heart disease have also in some cases,

in our opinion, wrongly assigned these normal rate

and rhythm patterns, when seen postoperatively, to

sinus node injury. We contend that these patterns

are normaal findings and equally may have been

present preoperatively.8

Intermittent prolongation of the PR interval >O-2O Si2 representing first degree atrioventricular

block was found in nine subjects studied and in-

cluded three children with Mobitz type I second

degree atrioventricular block. Mobitz type I atrio-

ventricular block has previously been described on

standard electrocardiograms in apparently healthy

young athletes'9 30 and in older men undergoing

strenuous exercise." In two large standard

electrocardiographic studies on asymptomatic adults

it was found in three of 67 375"3 and in two of

19 OOO,'4 respectively. The latter study showed

atrioventricular block in the supine position which

was abolished by standing up. His bundle and auto-

nomic function tests were performed in two male

adolescents being routinely examined.29 Electro-

physiological studies showed an area of block

proximal to the His bundle, and exercise and atro-

pine both abolished this conduction pattern. Intra-

cardiac electrophysiological and autonomic function

tests would have been of interest in our three sub-

jects with Mobitz type I atrioventricular block since

a recent report by Young et al." has shown that

seven of 16 children with this subsequently de-

veloped complete atrioventricular block and two

needed pacemaker treatment for syncope. One of the

three children with second degree atrioventricular

block in our study had suffered from four previous

fainting episodes but none had occurred for three

years. There was also a strong history of fainting in

her family. Finally, some of the reported studies on

atrioventricular block patterns in healthy subjects

without structural heart disease have suggested that

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the atrioventricular block is autonomic in originand does not represent a primary atrioventricularnodal conduction abnormality.29 30 A study byGambetta et al.36 convincingly showed that seconddegree atrioventricular block in two healthy adoles-cents was the result of a differential effect of vagaltone on the sinus and atrioventricular nodes.

Isolated premature beats were seen in 21 per centof children but none had more than one per hour.It is interesting to compare this finding with a24-hour electrocardiographic study of a populationof 134 healthy neonates with normal standardelectrocardiograms where 19 had premature beatsbut six had more than one an hour.37 In addition,14 of 2030 neonates on a standard electrocardio-gram alone have been shown to have multiplepremature beats. Eleven had ventricular, 14 supra-ventricular, and one had both.'8

In considering patterns of rhythm on surfaceelectrocardiography that should be regarded asnormal findings, it may be important to differentiatebetween the standard and the 24-hour electrocardio-gram. For example, at this age a constant heart rateof 40 a minute on a standard electrocardiogram isprobably abnormal because it implies that it is

J.1435

2'33

2 051

Table 6 Details on subjects with first or second degreeatrioventricular block on 24-hour electrocardiographicrecordings

Case no. First degree A V block Second degree AV block

PR interval (s) Number of episodes

1 0-24 53**2 0-24 5*3 0-24 94 0-205 0-246 0-247 0-248 0-209 0-28

**49 occurred during the day.*History of three faints.

present for a large part of the time, whereas a sthorepisode of bradycardia down to 40 a minute on a24-hour recording is probably normal. The assess-ment of the frequency and duration of rate orrhythm changes is therefore as relevant as theiroccurrence. It is, however, possible, by chance alone,that any arrhythmia detected by long-term electro-cardiographic recording may appear on a standardelectrocardiogram. Thus, arrhythmias which are

130/ min

44/m

'680 '720 '680 '700 680'700'680' 680'680'

Fig. 10 Twenty-four-hour electrocardiographic recording from a healthy 9-year-old girl. The top panel at 1435hours shows a sinus tachycardia of 130/minute with a normal PR interval. At other times during the recording thePR interval was 0-24 s. At 2133 hours and 2051 hours there are episodes of Mobitz type I (Wenckebach)atrioventricular block.

289

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normal on a 24-hour electrocardiogram could intheory also be normal on a standard electrocardio-gram. The findings of this study also suggests,therefore, that the range of normal for standardelectrocardiograms in children requires reappraisal.From the data in this paper we consider that the

following heart rate and rhythm patterns occur sofrequently in normal, healthy children that, thoughlong-term studies of their natural history have notbeen performed, they are probably not harmful.(1) Short episodes of bradycardia down to 37 a

minute and of tachycardia up to 195 a minute(within two standard deviations of the mean inthis r:ormal distribution of lowest and highestheart rates).

(2) Junctional escape rhythms of up to 20 minutes'duration (95th centile for maximum duration in24 subjects, Table 3).

(3) Sinus pauses containing PP interval patternspreviously described by other authors asevidence of abnormal sinus node function.7-10The maximum sinus pause should not exceed1 82 seconds (two standard deviations above themean longest sinus pause).

(4) Isolated superaventricular premature beats (nomore than one an hour).We remain hesitant in describing intermittent

first or second degree atrioventricular block as anormal, and therefore by inference a harmlessrhythm pattern. There is a significant incidence ofsudden unexpected death in adolescents, parti-cularly those undergoing competitive sport,39 andit is possible that children with an inbuilt tendencyto atrioventricular block are more at risk. Pros-pective long-term studies are required to answerthis important question.Ten per cent of children in this study had a

history of fainting but their incidence of rhythm orrate patterns was not significantly different fromthose children who did not faint. Though cardiacarrhythmias may cause a fall or cessation of cardiacoutput sufficient to produce cerebral hypoperfusionand hence syncope, autonomic changes may alsoproduce similar disturbances of cardiac conductiontogether with direct effects on venous or arterialtone. Moreover, isolated sinus node failure shouldnot result in asystole unless there is also a failureof the atrioventricular nodal or ventricular escapemechanism. How then should children who presentwith fits, faints, or dizziness be investigated?Causes outside the cardiovascular system, such asepilepsy, should first be excluded. Standard and24-hour electrocardiograms should be performedand examined for features outside the normal range.Carotid sinus massage or autonomic function testsmay help to establish a cardiovascular cause but

do not differentiate between vagal hypersensitivityand a primary abnormality of cardiac rhythm.40Intracardiac electrophysiological studies may also bevaluable in selected cases with abnormalities on thestandard or 24-hour electrocardiogram. Abnor-malities of sinus node function, however, may not beelicited by this technique.41 Ambulatory electro-cardiographic monitoring is a simple and safetechnique which may provide valuable informationin children suffering from attacks of dizziness orsyncope. Interpretation of recordings in sympto-matic children, however, requires knowledge of therange of heart rate and rhythm variations in normalchildren.

In conclusion, this study has shown that electro-cardiographic patterns previously reported toindicate abnormality of sinus node function occurfrequently in the healthy child. Similarly, the rangeof heart rates is wider than previously reported atthis age. It is essential that findings in normal,healthy children are taken into consideration wheninterpreting heart rate and rhythm patterns inclinical situations.

Dr Southall is a Senior Research Fellow with theBritish Heart Foundation. This project was fundedby the Foundation for the Study of Infant Deathsand the 24 hour electrocardiograph analyser wasprovided by the British Heart Foundation. Wewould like to acknowledge the help of Mr D Hall,Headmaster of St Osmund's School, Dorchester,and Dr K Adams and Mr F House of the Com-munity Medical Department, West Dorset HealthCare District.

References

1 Radford DJ, Izukawa T. Sick sinus syndrome.Symptomatic cases in children. Arch Dis Child 1975;50: 879-85.

2 Scott 0, Macartney FJ, Deverall PB. Sick sinussyndrome in children. Arch Dis Child 1976; 51:100-5.

3 Young D, Eisenberg RE. Symptomatic sinus arrestin a young girl. Arch Dis Child 1977; 52: 331-4.

4 Yabek SM, Jarmakani JM. Sinus node dysfunctionin children, adolescents and young adults. Pediatrics1978; 61: 593-8.

5 Yabek SM, Swensson RE, Jarmakani JM. Electro-cardiographic recognition of sinus node dysfunctionin children and young adults. Circulation 1977; 56:235-9.

6 Urbach JR, Pluvichit B, Zweizig H, et al. Instan-taneous heart rate patterns in newborn infants.Am 7 Obstet Gynecol 1965; 93: 965-74.

7 Myerburg RJ. Electrocardiographic diagnosis ofsinus node rhythm variations and SA block. In:Schlant RE, Willis-Hurst J, eds. Advances in

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Requests for reprints to Dr D P Southall, Depart-ment of Paediatrics, Cardiothoracic Institute,Fulham Road, London SW3 6HP.

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